设计模式学习（五）：单例模式

版权声明： https://blog.csdn.net/qq_29166327/article/details/82229348

2018年08月30日

目录

1、单例模式概念

2、饿汉模式

3、懒汉模式

---

1、单例模式概念

• 确保某一个类只有一个实例，并且自行实例化，并且向整个系统提供这个实例。
• 与全局变量的区别：全局变量不能实现继承，而单例模式可以；

2、饿汉模式

package designModel.SingleExampleModel;

public class testSingleExample {
	
	

	public testSingleExample() {}
	
	private static testSingleExample test = new testSingleExample();
	
	public static testSingleExample getInstance(){
		if(test == null){
			System.out.println("未实例化。。");
		}
		return test;
	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		System.out.println("1 "+testSingleExample.getInstance());
		Thread thread = new Thread(new Runnable(){

			@Override
			public void run() {
				// TODO Auto-generated method stub
				for(int i =1; i<11; i++){					
					System.out.println(i+" "+testSingleExample.getInstance());
				}
			}
			
		});
		thread.start();
	}

}

说明：饿汉模式，即对象实例化发生在编译时，static 静态变量被初始化了。

console：

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1 designModel.SingleExampleModel.testSingleExample@5ce345c2
1 designModel.SingleExampleModel.testSingleExample@5ce345c2
2 designModel.SingleExampleModel.testSingleExample@5ce345c2
3 designModel.SingleExampleModel.testSingleExample@5ce345c2
4 designModel.SingleExampleModel.testSingleExample@5ce345c2
5 designModel.SingleExampleModel.testSingleExample@5ce345c2
6 designModel.SingleExampleModel.testSingleExample@5ce345c2
7 designModel.SingleExampleModel.testSingleExample@5ce345c2
8 designModel.SingleExampleModel.testSingleExample@5ce345c2
9 designModel.SingleExampleModel.testSingleExample@5ce345c2
10 designModel.SingleExampleModel.testSingleExample@5ce345c2
 

可见上面获取到的对象都是同一个，单例模式效果。

3、懒汉模式

package designModel.SingleExampleModel;

public class testSingleExampleOfLazyType {

	public testSingleExampleOfLazyType() {}
	
	public static testSingleExampleOfLazyType test = null;
	
	public static testSingleExampleOfLazyType getInstance(){
		if(test == null){
			System.out.println("懒汉模式==实例化。。。");
			test = new testSingleExampleOfLazyType();
			return test;
		}else {
			return test;
		}
	}
	
	public static void main(String [] args){
		System.out.println(testSingleExampleOfLazyType.getInstance());
		
		Thread thread = new Thread(new Runnable(){

			@Override
			public void run() {
				// TODO Auto-generated method stub
				for(int i = 1;i<11;i++){
					System.out.println(i+" "+testSingleExampleOfLazyType.getInstance());					
				}
			}
			
		});
		
		thread.start();
		
	}
}

说明：懒汉模式即，对象的实例化（内存分配）发生在被调用的那一刻。

懒汉模式==实例化。。。
designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
1 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
2 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
3 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
4 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
5 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
6 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
7 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
8 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
9 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
10 designModel.SingleExampleModel.testSingleExampleOfLazyType@5ce345c2
 

只有当单例模式提供的外部接口被调用，对象才发生实例化。